Generally, the cargo space in commercial aircraft is inaccessible during flight. For this reason, most commercial aircraft rely on automatic fire-extinguishing systems to extinguish fires which occur in the cargo space and to keep the fire suppressed for the duration of the flight.
Most fire extinguishing systems for aircraft cargo spaces include two sources containing a fire extinguishing agent. The first source rapidly discharges the fire extinguishing agent to knock down the initial fire erupting within the cargo compartment. The second source releases the extinguishing agent at a much slower rate, and prevents fire from reigniting within the compartment. The rate of discharge is dependent on the size of the cargo space. Without the extended discharge, the concentration of the fire extinguishing agent in the cargo space could drop below what is necessary to keep the fire suppressed and embers could reignite the fire.
Previous fire extinguishing systems, such as U.S. Pat. No. 5,183,116 by Fleming, U.S. Pat. No. 5,083,867 by Hindrichs et al., and U.S. Pat. No. 4,643,260 by Miller, disclose the use of two independent fire extinguishers, consisting of two separate containers. Each container is equipped with its own charge valve, safety relief, pressure indicator, discharge outlet, explosive cartridge, rupture disc assembly, mounting lugs, and doublers. These types of systems take up excessive space, use longer piping connections, and require excessive time for installation onto the aircraft mounting platform.
Another type of fire extinguishing system is the "Bottle Within a Bottle" designed and manufactured by the Pacific Scientific Company in 1982. This design used an outer container as the high rate discharge fire extinguisher and an inner container as the metering fire extinguisher. While this system had several advantages over two separate extinguishers, relatively complicated weld structure joined the outer container and the inner container together to form a single fire extinguisher. The disadvantages of this design include the difficulty in monitoring the pressure of the inner container, the dependence of the outer container size on the size of the inner container, and the cost and complication of the design.
Thus, a need exists for a fire extinguishing system which not only is more compact, but also has fewer parts and increased reliability.